Method of varying the crystalline structure of or the concentration of impurities contained in a tubular starting crystal or both using diagonal zone melting

ABSTRACT

Method of varying the crystalline structure of or the concentration of impurities contained in a tubular starting crystal includes heating the tubular starting crystal to a temperature at which an annular molten zone is formed therein and passing the annular molten zone through the tubular starting crystal in the direction of the longitudinal axis thereof.

United States Patent Keller [4 Apr. 8, 1975 [5 METHOD OF VARYING THE2.471.437 5/1949 Lester 23/273 CRYSTALLINE STRUCTURE or OR THE 85:32? bg s an un a CONCENTRATION OF IMPURITIES 3.238.024 3/1966 Cremer et at23/301 CONTAINED IN A TUBULAR. STARTING 3.258.314 6/1966 Redmond et al.23/301 CRYSTAL 0R BOTH USING DIAGONAL 3.423.l89 1/1969 Pfann 23/301 ZONEMEL'UNG 3.454.367 7/1969 Reuschel 23/301 3.505.032 Bennett 23/30] [75]Inventor: Wolfgang Keller, Pretzfeld. Germany [73] Assignee: SiemensAlttiengesellschaft, Berlin and Munich. Germany [22] Filed: July 3],1972 [2]] Appl. No: 276,306

Related U.S. Application Data {63} Continuation of Ser. No. 872,279.Oct. 29. 1969.

abandoned.

[30] Foreign Application Priority Data Oct. 30, W68 Germany l80597l [52]U5. Cl 23/30l SP; 23/273 SP; 2l9/l0.43 [51] Int. Cl B0lj l7/l0 158]Field of Search 23/301 SP, 273 V. 273 SP; 2l9/l0.43

[56] References Cited UNITED STATES PATENTS l.892.lit)6 l/l933 Pederscn23/273 FOREIGN PATENTS 0R APPLICATIONS 75.36l 7/1954 Netherlands2l9/l0.43

Primary Examiner-Norman Yudkoff Assistant E.\'aminerR. T. FosterAttorney, Agent, or Firm-Herbert L. Lerner [57] ABSTRACT Method ofvarying the crystalline structure of or the concentration of impuritiescontained in a tubular starting crystal includes heating the tubularstarting crystal to a temperature at which an annular molten zone isformed therein and passing the annular molten zone through the tubularstarting crystal in the direction of the longitudinal axis thereof.

7 Claims. 8 Drawing Figures METHOD OF VARYING THE CRYSTALLINE STRUCTUREOF OR THE CONCENTRATION OF IMPURITIES CONTAINED IN A TUBULAR STARTINGCRYSTAL OR BOTH USING DIAGONAL ZONE MELTING This is a continuation, ofapplication Ser. No. 872,279, filed Oct. 29, I969, and now abandoned.

My invention relates to method of varying the crystalline structure of atubular starting crystal or the concentration of impurities containedtherein or both.

Tubular monocrystals of semiconductor material, such as siliconparticularly, having considerable length such as 50 cm, for example, andconsiderable diameter such as 25 cm, for example, as well as arelatively thin wall thickness, such as mm, for example, are required,for example, in Rontgen spectroscopes. Such tubular monocrystals can beproduced by the method in copending application Ser. No. 872,278, forexample, which has been simultaneously filed with the hi stantapplication and of which l am a coinventor. Specific concentrations ofimpurities is the tubular monocrystal are also desirable.

Such tubular monocrystals can be produced by mechanical processing fromrod-shaped monocrystals previously obtained by crucible-free zonemelting. A central opening must be bored through the rod-shapedmonocrystal in the axial direction thereof. This operation is verycostly and time-consuming especially for relatively long and thickrod-shaped monocrystals because of the attendant danger that the wall ofthe hollowed-out tubular monocrystal will burst or collapse due tocrack-formation therein, unless great care is taken in performing theboring operation. Furthermore, the tubular monocrystal becomescontaminated by the material of the boring tool and by the coolantemployed during the boring operation. In addition, the diameter ofrod-shaped monocrystals produced by cruciblefree, floating-zone meltingor by pulling from a melt contained in a crucible is limited to amaximum of about 7.5 cm.

It is an object of my invention to provide method of varying thecrystalline structure of or the concentration of impurities contained ina tubular starting crystal or both which avoids the aforementioneddifficulties of heretofore known methods of producing tubularmonocrystals.

With the foregoing and other objects in view, I pro vide, in accordancewith my invention, method of varying the crystalline structure of or theconcentration of impurities contained in a tubular starting crystal,which comprises heating the tubular starting crystal to a temperature atwhich an annular molten zone is formed therein, and passing the annularmolten zone through the tubular starting crystal in the direction of thelongitudinal axis thereof.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as method ofvarying the crystalline structure of or the concentration of impuritiescontained in a tubular starting crystal or both, it is nevertheless notintended to be limited to the details shown, since various modificationsmay be made therein without departing from the spirit of the inventionand within the scope and range of equivalent of the claims.

The invention, however, together with additional ob jects and advantagesthereof, will be best understood from the following description whenread in connection with the accompanying drawings in which:

FIG. 1 is a longitudinal view, partly in section, of a tubular crystaland apparatus for passing a molten zone therethrough in accordance withthe method of my invention;

FIG. 2 is a cross-sectional view of FIG. I taken along the line II II inthe direction of the arrows;

FIG. 3 is another view similar to that of FIG. 1 showing modifiedapparatus for performing another mode of the method of the inventionperformed by the apparatus of FIG. 1;

FIG. 4 is a perspective view of the heating device forming part of theapparatus of FIG. 3;

FIGS. 5 and 5a are top plan and perspective views, respectively, of aspecial heating device for carrying out the method of my inventionaccording to FIGS. 1 and 3', and

FIGS. 6 and 7 are longitudinal sectional views ofa tubular crystal andapparatus for performing another mode of the method of my invention.

Referring now to the drawings, and first particularly to FIGS. 1 and 2thereof, there is shown a tubular starting crystal 101 having a circularcross section and formed of silicon. Such tubular starting crystals,which can either have varying cross sections or uniformly equal crosssection over the entire length thereof. can be produced by boringthrough a polycrystalline rod produced by precipitation or deposition ofsemiconductor material as a result of a reaction of a gaseoussemiconductor compound with a heated rod-shaped carrier member of thesame semiconductor material as described in German Pat. No. 1,061,593.Such tubular starting crystals can also be produced by the method of theaforementioned copending application simultaneously filed with theinstant application which discloses a method of precipitating a layer ofsemiconductor material from a gaseous compound of the semiconductormaterial, wherein I deposit on the outer surface ofa heated cylindricalcarrier member ofa different resistant material a layer of semiconductormateral of predetermined thickness from a gaseous compound of thesemiconductor material and, after the semiconductor layer has beendeposited, mechanically or chemically removing the semiconductor layerwithout destroying or damaging the shape thereof.

The starting crystal 101, as shown in FIG. I, is secured in an endholder 102 by three screws 103 spaced circumferentially at equal anglesfrom one another. In the interest of clarity, the holder 102 and thestarting crystal 101 are shown partly broken away and in section in FIG.1, and only two of the three screws I03 are visible in the figure.

A relatively thin monocrystalline seed crystal 107, which is, in turn,fastened at one end to a holder 108, is fused at the other end thereofto a part I06 of the lower end surface, as shown in FIG. 1, of thetubular starting crystal 101. The tubular crystal 101 is surrounded byan elliptical heating device I04, whose shape is seen more clearly inFIG. 2 which is a cross section of FIG. 1, taken at the level of theheating device 104. the heating device 104 of FIGS. I and 2 is in theform of a coil having a single winding supplied with high frequencyalternating current but may also be any suitable electrically heatedradiant heating device. The

heating device 104 heats a molten zone 105 formed in the tubularstarting crystal 101 in the shape of ellipse, as shown in FIG. 2.

The elliptical molten zone 105 is disposed in a plane which is neitherparallel nor perpendicular to the axis 109 of the tubular startingcrystal 101. Starting from the location 106 at which the rod-shaped seedcrystal 107 is fused to the lower surface of the tubular startingcrystal 101, as shown in FIG. 1, the molten zone 105 is passed at leastonce through the starting crystal 101, in accordance with the methodofinvention. Ifthe concentration distribution of impurities contained inthe starting crystal 101 is to be varied, several passes of the moltenzone 105 through the crystal 101 may be required.

For this purpose, the tubular starting crystal 101 and the heatingdevice 104 of elliptical shape. are moved relative to one another in therelative direction of the arrow 112 so that the axis of the startingcrystal 101 is inciined at an angle varying between and 90 with respectto the major axis 110 of the elliptical heater winding 104, as shown inFIG. I, and is perpendicular to the minor axis 111 of the ellipticalwinding 104 as shown in FIG. 2. The angle of inclination of the axis 109to the major axis 110 of the ellipse is advanta geousiy within the rangeof to 65 and is preferably 45.

The heating device can also be in the form of a flat coil of ellipticalcross section, having several windings disposed in one plane or a coilwith several elliptical windings in the shape of an inclined cylinderhaving elliptical top and bottom end surfaces. both of the coilssurrounding the tubular starting crystal and being supplied withhigh-frequency alternating current. In the case of such coils also, thetubular starting crystal 101 and the coil are relatively displaced sothat the axis 109 of the crystal 101 is inclined at an angle between 0and 90 with respect to the major axis of the elliptical cross section ofthe coil and is perpendicular to the minor axis of the ellipse.

Due to this inclination of the axis 109 of the tubular starting crystal101 to the major axis of the elliptical cross section of the heatingdevice 104, assurance is provided that the material in the wall of thetubular starting crstyal 101 recrystallizes uniformly and as amonocrystal when the molten zone 105. starting from the location 106 atwhich the monocrystalline seed crystal 107 is fused thereto. is passedtherethrough.

An electron beam or a plasma discharge are also suited for heating themolten zone 105. Furthermore. it may also be advantageous to disposeadditional heat ing devices. such as coils or radiant heaters energizedby highfrequency alternating current, above or below or both above andbelow the heating device 104, so as to thereby respectively preheat thetubular starting crystal I01 prior to the molten zone pass therethroughand/or postheat the recrystallized monocrystal after the molten zonepass therethrough. The dislocation density in the recrystallizedmonocrystalline material can thereby be especially reduced.

It is desirable to dispose the heating device 104 in a horizontal planeso that the molten zone 105 is also located in a horizontal plane.whereby assurance is provided that the molten semiconductor materialwill not drip out of the molten zone 105 during passage of the moltenzone through tubular starting crystal 101.

To vary the wall thickness of the tubular starting crystal 101, thespacing between the ends of this crystal 101 can be varied durng themolten zone pass therethrough. To thicken the wall of the tubularstarting crystal 101, the holders 102 and 108 and accordingly the endsof the starting crystal 101 are displaced in the axial direction towardone another during the zonemelting pass. The tubular wall of the crystal101 is correspondingly made thinner by displacing the holders 102 and108 and accordingly the ends of the tubular crystal 101 away from oneanother in the axial direction of the crystal 101.

In the apparatus embodiment of FIG. 3, wherein like elements areidentified by the same reference numerals as in FIGS. 1 and 2, thetubular starting crystal 101 and an annular heating device 304 are alsodisplaceable relative to one another. The heating device 304 is provided with relatively long current-supply leads 312 and is disposedwithin the tubular starting crystal 101 for heating the molten zone 105.

As shown in FIG. 4, the heating device 304 is in the form of an ellipse.The inclination of the axis 109 of the starting crystal 101 to the minorand major axes 310 and 311 of the elliptical cross section of theheating device 304 is the same as for the heating device 104 of FIG. 1.

The heating device 304 can also be a flat or cylindri cal coil havingelliptical cross section or an electrically heated radiant heater in theform of an ellipse, all energized with high-frequency alternatingcurrent.

The molten zone being passed through the tubular starting crystal canalso be heated by two coils supplied with high-frequency alternatingcurrent, one of the coils being disposed inside the tubular startingcrystal and the other of the coils surrounding the outside of thestarting crystal. Both of such coils are preferably singly ormultiply-wound flat coils of elliptical cross section which lie in thesame plane. Both cells are connected serially in opposite phase i.e.,they are transversed by energizing current flowing in opposite directiontherethrough. FIG. 5 shows in top plan view, two of such oppositelyphased serially connected coils 513 and 514 having a single winding.respectively, and formed with an elliptical cross section. The coils 513and 514 are disposed in a common plane and, as shown in FIG. 5, the wallof the tubular starting crystal 501, whose cross section is in the formof a circular ring, passes therebetween. The opposite current flowthrough both coils S13 and 514 is represented by the arrows 516 and 517.In the interest of clarity, only the cross section of the part of thestarting crystal 501 actually located between the two coils 513 and 514is shown in broken lines in FIG. 5. In the interspace between the twocoils 513 and 514, the field strength of the electromagnetic fieldemanating from the coils 513 and 514 has a maximum value. The currentleads 515 to the induction heating coil 514 located in the interior ofthe tubular starting crystal 501 are advantageously bent into anelongated U-shaped bow between the legs of which the tubular startingcrystal S01 is located in the course of the relative movement thereofduring the molten zone pass through the crystal 501. The spacing of thisU-shaped bow from the tubular starting crystal 501 is suitablysufficiently great enough so that the bow does not act as an additionalheat device. The inclination of the axis of the tubular starting crystal501 with respect to the major and minor axes of the elliptical crosssection of both coils 513 and 514 is the same as for the mode of themethod of my invention illustrated in FIG. 1.

For a clearer understanding of the construction of the coils 513 or 514of FIG. 5, a perspective view thereof is shown in FIG. 5a and isbelieved to require no further explanation.

FIG. 6 illustrates another mode of the method of my invention, whereinthere is processed a tubular starting crystal 601 having a circularcross section and conically tapering end to which a monocrystalline seedcrystal 607 is fused. The starting crystal 601 can also be produced bythe method of the aforementioned copending application simultaneouslyfiled with this application, wherein a suitably shaped carried member isemployed.

The tubular starting crystal 601 is secured in a holder 602 by means ofscrews 603 so that the axis 609 of the crystal 601 extends in verticaldirection. A tubular monocrystalline seed crystal 607, which is fastenedat one end thereof to a holder 608, is fused at the other end thereof tothe face of the lower conically tapering end of the tubular startingcrystal 601 by an annular heating device 604 of circular cross section.The tubular seed crystal 607 can be produced relatively simply, forexample. by partially boring a longitudinal recess in a relatively thinrod-shaped monocrystal, for example of 44 mm diameter. A molten zone 605is passed through the tubular starting crystal 601 by relativedisplacement of the tubular starting crystal 601 and the heating device604, as shown further in FIG. 7, in axial direction of the crystal 601,beginning from the location at which the starting crystal 601 and theseed crystal 607 are fused to one another. The material recrystallizingfrom the molten zone 605 after it has passed through the polycrystallinestarting crystal 601 is monocrystalline. The holders 602 and 608 andaccordingly both ends of the tubular starting crystal 601 can be set inrelative rotation about the axis of the tubular crystal 601 during themolten zone pass through the crystal 601. The recrystallized tubularmonocrystal that is thereby produced has a uniformly round circularcross section. The heating device 604 can also be in the form of a coilhaving one or more windings or an electrically heated radiant heatersupplied with highfrequency alternating current.

The thickness of the tubular wall of the starting crystal 601 shown inFIGS. 6 and 7 can also be increased by reducing the axial spacingbetween the holders 602 and 608, and the wall thickness of the crystal601 can conversely be reduced by increasing the spacing between theseholders 602 and 608.

1 claim:

1. Method of producing a monocrystalline tube of semiconductor materialwhich comprises fusing to a part of the edge of one end of apolycrystalline tube of semiconductor material a monocrystalline seedcrystal of the semiconductor material having a diameter that isconsiderably smaller than the diameter of the tube, positioning the tubeso that the axis thereof is oblique and the junction of the seed crystaland the tube is at the lowest point on the tube and sequentiallyexposing the tube from the bottom upwards to an essentially horizontalheat zone essentially coaxial with the tube and at a temperature atleast as high as the melting point of the tube thereby first to melt thetube at and adjacent to said junction and thereafter move an essentiallyhorizontal annular molten zone upwards along the tube.

2. The method of claim 1, wherein the heat zone is produced by a coilhaving an elliptical cross section, said coil surrounds the tube and isenergized by highfrequcncy, alternating current.

3. The method of claim 1, wherein a radiant heater of elliptical crosssection surrounding said tube is used for producing the heat zone.

4. The method of claim 1, wherein a radiant heater of elliptical crosssection, situated in the interior of the tube, is used for producing theheat zone.

5. The method of claim 1, wherein the heat zone is produced by twooppositely phased series-connected coils, one of said coils beingsituated inside and the other of said coils being situated outside saidtube.

6. The method of claim 1, in which the inclination of the axis of thetube from the horizontal is from 25 to 65.

7. The method of claim 6, in which the inclination of

1. METHOD OF PRODUCING A MONOCRYSTALLINE TUBE OF SEMICONDUCTOR MATERIALWHICH COMPRISES FUSING TO A PART OF THE EDGE OF ONE END OF APOLYCRYSTALLINE TUBE OF SEMICONDUCTOR MATERIAL A MONOCRYSTALLINE SEEDCRYSTAL OF THE SEMICONDUCTOR MATERIAL HAVING A DIAMETER THAT ISCONSIDERABLY SMALLER THAN THE DIAMETER OF THE TUBE, POSITIONING THE TUBESO THAT THE AXIS THEREOF IS OBLIQUE AND THE JUNCTION OF THE SEED CRYSTALAND THE TUBE IS AT THE LOWEST POINT ON THE TUBE AND SEQUENTIALLYEXPOSING THE TUBE FROM THE BOTTOM UPWARDS TO AN ESSENTIALLY HORIZONTALHEAT ZONE ESSENTIALLY COAXIAL WITH THE TUBE AND AT A TENEMPERATURE ATLEAST AS HIGH AS THE MELTING POINT OF THE TUBE THEREBY
 2. The method ofclaim 1, wherein the heat zone is produced by a coil having anelliptical cross section, said coil surrounds the tube and is energizedby high-frequency, alternating current.
 3. The method of claim 1,wherein a radiant heater of elliptical cross section surrounding saidtube is used for producing the heat zone.
 4. The method of claim 1,wherein a radiant heater of elliptical cross section, situated in theinterior of the tube, is used for producing the heat zone.
 5. The methodof claim 1, wherein the heat zone is produced by two oppositely phasedseries-connected coils, one of said coils being situated inside and theother of said coils being situated outside said tube.
 6. The method ofclaim 1, in which the inclination of the axis of the tube from thehorizontal is from 25.degree. to 65.degree..
 7. The method of claim 6,in which the inclination of the axis of the tube from the horizontal is45.degree..